[Haskell-beginners] Trying to understand type families.

For this question, I'll refer to http://www.haskell.org/haskellwiki/GHC/Type_families#A_unit_instance In particular this example instance GMapKey () where data GMap () v = GMapUnit (Maybe v) empty = GMapUnit Nothing lookup () (GMapUnit v) = v insert () v (GMapUnit _) = GMapUnit $ Just v Could someone explain what the () is doing?